Certain stages of semiconductor manufacturing require thermal cycling of the semiconductor substrate wherein the substrate is heated and then chilled. For example, the photoresist processing stage of semiconductor manufacturing requires heating to flow the photoresist material along the substrate surface, followed by cooling or chilling the substrate to set the photoresist. In order to produce high quality wafers suitable for state of the art integrated circuit applications, the temperature of the wafer during thermal cycling must be precisely controlled with respect to both the temporal temperature profile of the thermal cycles and the uniformity of the temperature across the substrate.
Conventional methods for heating and chilling wafers involve first baking the wafer at a temperature ranging typically between 70.degree. C. and 250.degree. C. for a period of time ranging typically between 30 seconds and 90 seconds. After baking the wafer, the wafer is then mechanically moved to a cold plate where it is chilled to a temperature ranging typically between 0.degree. C. and 30.degree. C. Disadvantages of the conventional methods for heating and chilling wafers include the inability to control temperature variations at the wafer surface during the transfer between plates and/or between processing stations, and the potential for wafer damage from physical mishandling or contamination during transfer.
Recent developments for heating and chilling wafers involve thermal cycling modules where a wafer is heated and chilled on one plate, thus eliminating the need to move the wafer between plates. In such a thermal cycling module, the wafer is placed on a thermal conduction plate which is thermally coupled to a heating and cooling device. The heating and cooling device is controlled by a controller which is programmed with the desired thermal cycling profile (e.g., heating to 80.degree. C. for 10 seconds, increase temperature to 200.degree. C. for 10 seconds, followed by cooling to 10.degree. C. for 30 seconds, etc.) for processing the wafer.
To achieve precise temperature control at the wafer, a plurality of sensors (e.g., thermocouple sensors or infrared sensors) are generally connected as described in parent application Ser. No. 08/939,926, to provide feedback to the controller regarding the temperature at the wafer surface. While the programmed thermal cycling profile for a given wafer may have fixed temperature and duration values, the wafer temperature ramp rates for different stages of the thermal cycling profile, for different wafer-bearing plates, and for the processing of different wafers, can vary drastically resulting in quality variations in the wafers processed thereby.
Accordingly, a need exists for an improved control system for controlling wafer temperature ramp rates during processing within thermal cycling systems.